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The family of a four-year-old Australian girl whose rare brain tumour has been shrunk by experimental treatment in Mexico can only afford to return for the potentially lifesaving therapy one more time.

Annabelle Potts, from Canberra, was given nine months to live when she was diagnosed with a rare type of brain cancer called Diffuse Intrinsic Pontine Glioma (DIPG) on Boxing Day in 2016.

Her parents, Kathie and Adam, feared they would lose their toddler, whose speech, behaviour and ability to walk had already deteriorated.

Unwilling to accept the diagnosis, the couple searched the internet for treatments that could potentially give Annabelle hope of seeing her next birthday and found a minimally invasive option in Mexico.

"This will be her sixth treatment. Annabelle and Adam left for Mexico yesterday," Ms Potts told nine.com.au.

Annabelle was nervous to travel back to Mexico for her sixth treatment, but Ms Potts said she loves the plane and visiting friends in the hospital. (Supplied)

The couple and their son, William, initially relocated to Mexico with Annabelle so she could be treated every 17 days.

But three months in doctors noticed her tumour had been shrinking at each scan and they decided she could push her treatment to every six weeks.

Annabelle was only three-years-old when she was diagnosed with Diffuse Intrinsic Pontine Glioma. (Love For Annabelle)

"At least in Mexico they are giving it a go and with this type of tumour that's what we have to do or risk losing her."

Ms Potts said Annabelle has been in good spirits, but can be quite distressed when she comes out of the treatment.

"She has to go under a general anaesthetic for two hours then wakes up with needles and chords sticking out of her.

"She gets really emotional which is really hard. We just have to hug and kiss her until she is okay."

Ms Potts said the treatment has been used for several years to treat other types of cancer in Mexico, but is experimental with Annabelle's particular tumour.

Young Annabelle endured relentless vomiting, slurred speech and extreme behavioural changes before she went to Mexico.

Ms Potts said the only side effect Annabelle has suffered is that she tiers easily. (Love For Annabelle)

"She was having major meltdowns about twice a day. I had never seen her have violent outbursts like that before," the Canberra mum said.

"We couldn't help her and we really struggled with it."

After only two days on the experimental treatment – which sees a catheter inserted through an artery in her groin and fed into her brain so the tumour can be targeted – Annabelle's behaviour returned to normal.

"She was vomiting every single morning because of the pressure of the tumour then days later nothing," Ms Potts said.

"The treatment also doesn't have the traditional side effects of chemotherapy like hair loss, nausea or a compromised immune system."

There are two other Australian families with a child receiving the same treatment in Mexico, according to Ms Potts.

She said some children are seeing their tumours shrink by up to 50 percent, while one boys has completely vanished.

"His tumour is 100 percent gone – he is the first one and it gives us so much hope for Annabelle," she said.

The Potts family have struggled to finance Annabelle's expensive treatments. (Love For Annabelle)

But tales of those who could no longer afford treatment and returned home to only to pass away leaves Ms Potts feeling "constantly on edge".

"There was one girl who was living over there but her family ran out of money and had to come back to Australia," she said.

"She was improving in Mexico but this type of tumour comes back so quickly, so aggressively, and when she got home she died."

Ms Potts said she was "absolutely gutted" to learn the "inspirational" young girl didn’t pull through.

"I feel for them because her mum was one of the first people we were in contact with about the treatment."

More than $250,000 has been raised to help Annabelle receive the life-changing therapy, but those resources have already been drained.

"We are incredibly lucky to be supported but we do need more help. We hate asking, but it's our daughter's life.

"Annabelle's tumour could come back at any time. She could wake up tomorrow and not be able to walk, or have her eye turned in."

Annabelle having her final immunotherapy and her 5th Intra-Arterial treatment before returning to Australia. (Love for Annabelle)

Ms Potts said it is hard enough living in constant fear of losing Annabelle, but having to shield her from the reality of her condition has made it far more difficult.

"She knows she has a lump in her head and if it comes back she might start getting dizzy and vomiting again," Ms Potts said.

"But she doesn't know if it does come back she is going to die."

The web designer said she had no idea brain cancer was the number one killer of children in Australia until Annabelle's diagnosis.

"Brain cancer loses out to all the other cancers that are a lot more well known, like brain cancer or leukaemia. More people are affected but the survival rate is much better."

Over five years, the breast cancer survival rate is 90 per cent while brain cancer like Annabelle's is less than one percent.

"People need to be more aware brain cancer is what is killing our children now. This is what we need to focus on," Ms Potts said.

If Annabelle’s tumour was to start growing again, Ms Potts said they would pick up their lives and move straight back to Mexico if they could afford it.

"There is nothing they can do here except put her on steroids and that is just to prolong her life. It would give her terrible symptoms and quality of life."

Ms Potts said having their family torn apart every six weeks so Annabelle can go for treatment is "heartbreaking".

"You want to be able to spend as much time as possible with her, even being away from her for a week is too long," she said.

The four-year-old is due to have treatment this morning and will return to Australia with her father on Sunday.